P-waves and s-waves make up a group called body waves, which travels through rock. The fastest are p-waves (primary, or compression waves), which squeeze rocks as they race through them. The movement of these waves is akin to someone in the back of a line shoving the person in front of them. As p-waves pass through different materials, they speed up and slow down, and that changes their path. So, by mapping how these p-waves change direction, scientists can attempt to piece together a planet’s internal layering and composition.
The other type of body wave is called an s wave (secondary, or shear wave), and these are the second fastest type of seismic wave. S-waves move rock perpendicular to the direction the wave is moving, like shaking a beach towel to fling off sand. In order to propagate, S-waves require material that has some internal strength and rigidity. This means that s-waves can’t pass through liquids, including our planet’s liquid metallic outer core.
The last two seismic waves make up a group called surface waves. On Earth, these are the waves that cause the most devastation. The first type, Love waves, oscillate the surface from side to side (horizontally). Meanwhile, the other type, Rayleigh waves, move through the surface like an ocean wave (vertically). When we feel the ground shaking during an earthquake, Love waves and Rayleigh waves are usually what we’re feeling.
inside looking out
Inside Earth, p- and s-waves speed up as you get deeper into the planet’s crust and mantle. There are some variations as the waves transition from the brittle crust to the malleable mantle to the dense core, but in general, they travel faster at greater depth. This is due to the changing pressure (and thus density) of Earth’s interior, as well as changes in the minerals that make up the mantle.
When the seismic waves hit Earth’s mantle-core boundary, big things happen. First, the s-waves disappear because the outer core of Earth is liquid. Remember, s-waves shear rocks, and with nothing to shear, they vanish. However, the s-waves return in the planet’s solid inner core. That’s because s-waves, or secondary waves, are mostly a byproduct of the movement of p-waves, or primary waves.
Meanwhile, p-wave velocities drop dramatically at the mantle-core boundary. That’s because they are going from silica-rich rocks to an iron-nickel core. In fact, inside Earth, p-waves drop in velocity by almost 50 percent. However, p-waves again speed up as they travel deeper into Earth’s outer and inner core.
After both s- and p-waves make their way through Earth’s core, their speeds change again — but this time in reverse, as they are now venturing from core out to crust.
In the shadows
Planets and moons are spheres. So, whether an earthquake, marsquake, or moonquake, a quake will send seismic waves that move in an arc-like shape through the world’s interior (see the image below). When these waves hit different materials, they increase or decrease in speed, causing their paths to bend. This phenomenon is known as refraction, and it’s the same thing that happens when light (which can be a wave) passes through a lens.